Plate wave devices with wave confinement structures and fabrication methods

ABSTRACT

A micro-electrical-mechanical system (MEMS) guided wave device includes a single crystal piezoelectric layer and at least one guided wave confinement structure configured to confine a laterally excited wave in the single crystal piezoelectric layer. A bonded interface is provided between the single crystal piezoelectric layer and at least one underlying layer. A multi-frequency device includes first and second groups of electrodes arranged on or in different thickness regions of a single crystal piezoelectric layer, with at least one guided wave confinement structure. Segments of a segmented piezoelectric layer and a segmented layer of electrodes are substantially registered in a device including at least one guided wave confinement structure.

This application is a continuation of U.S. patent application Ser. No.16/544,279 filed on Aug. 19, 2019 and issuing as U.S. Pat. No.11,545,955, which is a continuation of U.S. patent application Ser. No.14/973,336 filed on Dec. 17, 2015 and issuing as U.S. Pat. No.10,389,332, which is a continuation of International Patent ApplicationNo. PCT/US15/066424 filed on Dec. 17, 2015, which is a non-provisionalof U.S. Provisional Patent Application No. 62/093,184 filed on Dec. 17,2014, and is a non-provisional of U.S. Provisional Patent ApplicationNo. 62/093,753 filed on Dec. 18, 2014. The entire contents of theforegoing applications and patent are hereby incorporated by referenceas if set forth fully herein.

TECHNICAL FIELD

The present disclosure relates to electromechanical components utilizingacoustic wave propagation in piezoelectric layers, and in particular toimproved plate wave structures and methods for making such structures.Such structures may be used, for example, in radio frequencytransmission circuits, sensor systems, signal processing systems, andthe like.

BACKGROUND

Micro-electrical-mechanical system (MEMS) devices come in a variety oftypes and are utilized across a broad range of applications. One type ofMEMS device that may be used in applications such as radio frequency(RF) circuitry is a MEMS vibrating device (also known as a resonator). AMEMS resonator generally includes a vibrating body in which apiezoelectric layer is in contact with one or more conductive layers.Piezoelectric materials acquire a charge when compressed, twisted, ordistorted. This property provides a transducer effect between electricaland mechanical oscillations or vibrations. In a MEMS resonator, anacoustic wave may be excited in a piezoelectric layer in the presence ofan alternating electric signal, or propagation of an elastic wave in apiezoelectric material may lead to generation of an electrical signal.Changes in the electrical characteristics of the piezoelectric layer maybe utilized by circuitry connected to a MEMS resonator device to performone or more functions.

Guided wave resonators include MEMS resonator devices in which anacoustic wave is confined in part of a structure, such as in thepiezoelectric layer. Confinement may be provided by reflection at asolid/air interface, or by way of an acoustic mirror (e.g., a stack oflayers referred to as a Bragg mirror) capable of reflecting acousticwaves. Such confinement may significantly reduce or avoid dissipation ofacoustic radiation in a substrate or other carrier structure.

Various types of MEMS resonator devices are known, including devicesincorporating interdigital transducer (IDT) electrodes and periodicallypoled transducers (PPTs) for lateral excitation. Examples of suchdevices are disclosed in U.S. Pat. Nos. 7,586,239, 7,898,158, and8,035,280 assigned to RF Micro Devices (Greensboro, N.C., USA), whereinthe contents of the foregoing patents are hereby incorporated byreference herein. Devices of these types are structurally similar tofilm bulk acoustic resonator (FBAR) devices, in that they each embody asuspended piezoelectric membrane. Such devices (including IDT-typedevices in particular) are subject to limitations of finger resistivityand power handling due to poor thermal conduction in the structures.Additionally, IDT-type and PPT-type membrane devices may requirestringent encapsulation, such as hermetic packaging with a near-vacuumenvironment.

Plate wave (also known as lamb wave) resonator devices are also known,such as described in U.S. Patent Application Publication No.2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surfaceacoustic wave (SAW) devices, plate wave resonators may be fabricatedatop silicon or other substrates and may be more easily integrated intoradio frequency circuits. Reinhardt discloses a multi-frequency platewave type resonator device including a silicon substrate, a stack ofdeposited layers (e.g., SiOC, SiN, SiO₂, and Mo) constituting a Braggmirror, a deposited AlN piezoelectric layer, and a SiN passivationlayer. According to Reinhardt, at least one resonator includes adifferentiation layer arranged to modify the coupling coefficient of theresonator so as to have a determined useful bandwidth. One limitation ofReinhardt's teaching is that deposition of AlN piezoelectric material(e.g., via epitaxy) over an underlying material having a very differentlattice structure generally precludes formation of single crystalmaterial; instead, lower quality material with some deviation fromperfect orientation is typically produced. A further limitation is thatReinhardt's approach does not appear to be capable of producingresonators of widely different (e.g., octave difference) frequencies ona single substrate. Additionally, in at least certain contexts, it maybe cumbersome to produce Bragg mirrors with consistently highreproducibility of layer thicknesses.

Accordingly, there is a need for guided wave devices that can beefficiently manufactured. Desirable devices would address thermalconduction and stringent packaging concerns associated withmembrane-type devices. There is a further need to provide devices thatmay incorporate high quality piezoelectric materials. There is a stillfurther need for devices that may enable production of widely differentfrequencies on a single substrate.

SUMMARY

The present disclosure provides a micro-electrical-mechanical system(MEMS) guided wave device that utilizes a single crystal piezoelectriclayer and at least one guided wave confinement structure configured toconfine a laterally excited wave in the single crystal piezoelectriclayer. One or multiple guided wave confinement structures may beprovided. One guided wave confinement structure may include a Braggmirror separated from a single crystal piezoelectric layer by a slowwave propagation layer or a temperature compensation layer. Anotherguided wave confinement structure may include a fast wave propagationmaterial. Single crystal piezoelectric materials (e.g., lithium niobate,lithium tantalate, and the like) may be incorporated in such devices,such as by pre-fabrication followed by bonding to at least oneunderlying layer of a guided wave device to form a bonded interface.Multiple electrodes arranged in or on the single crystal piezoelectriclayer are configured for transduction of a lateral acoustic wave.

Embodiments incorporating fast wave propagation materials to provideguided wave confinement may benefit from ease of fabrication as comparedto production of Bragg mirrors in certain contexts. One or more Braggmirrors may be used in certain embodiments, such as may be useful totailor wave reflection parameters, and as also may be useful in thecontext of confining very high velocity acoustic waves. Certainembodiments incorporate a fast wave propagation material on or adjacentto one (e.g., a first) surface of a piezoelectric material, andincorporate a Bragg mirror adjacent to another (e.g., a second) surfaceof the piezoelectric material.

Guided wave devices incorporating various electrode configurationsdisclosed herein include, but are not limited to, single layer coplanarinterdigital transducers (IDTs) alone, multiple layer coplanar IDTsalone, IDTs in combination with continuous layer electrodes (e.g.,useable as floating electrodes or shorting electrodes to enable launchof asymmetric waves), IDTs at least partially embedded in piezoelectriclayers, non-coplanar IDTs, IDTs registered with single crystalpiezoelectric layer segments, and periodically poled transducers (PPTs).In certain embodiments, electrodes may be partially embedded in, and/orgaps between various electrodes may be filled in whole or in part with,(i) piezoelectric material, or (ii) slow wave propagation materialand/or temperature compensation material. The wavelength A of anacoustic wave transduced by an IDT equals two times the pitch orseparation distance between adjacent electrodes (fingers) of oppositepolarity, and the wavelength A also equals the separation distancebetween closest electrodes (fingers) of the same polarity.

In certain embodiments, MEMS guided wave devices employ single-sidedconfinement, in which at least one confinement structure is providedadjacent to a first surface of a single crystal piezoelectric layer, andin which a solid/air interface is provided adjacent to a second opposingsurface of the single crystal piezoelectric layer. In other embodiments,MEMS guided wave devices employ double-sided confinement, in which firstand second confinement structures are provided proximate to first andsecond opposing surfaces, respectively, of a single crystalpiezoelectric layer.

In one aspect, a MEMS guided wave device includes multiple electrodesarranged in or on a single crystal piezoelectric layer and configuredfor transduction of a lateral acoustic wave in the single crystalpiezoelectric layer. At least one guided wave confinement structureincludes a Bragg mirror proximate to the piezoelectric layer, whereinthe Bragg mirror is configured to confine a laterally excited wave inthe single crystal piezoelectric layer, and the Bragg mirror isseparated from the single crystal piezoelectric layer by a slow wavepropagation layer. In certain embodiments, the Bragg mirror includes atleast one group of at least one low impedance layer and at least onehigh impedance layer, and the at least one low impedance layer issequentially arranged with the at least one high impedance layer in theat least one group. In certain embodiments, the laterally excited wavein the single crystal piezoelectric layer has a wavelength A, and eachguided wave confinement structure of the at least one guided waveconfinement structure comprises a thickness of less than 5λ. In certainembodiments, a bonded interface is provided between the single crystalpiezoelectric layer and at least one underlying layer of the device(such as a guided wave confinement structure, or a slow wave propagationlayer, or a substrate).

In certain embodiments, a single crystal piezoelectric layer includes afirst surface and a second surface opposing the first surface, the atleast one guided wave confinement structure includes a first guided waveconfinement structure proximate to the first surface and includes asecond guided wave confinement structure proximate to the secondsurface. In certain embodiments, a first guided wave confinementstructure includes a first Bragg mirror, and a second guided waveconfinement structure includes either a fast wave propagation materialor a second Bragg mirror. In certain embodiments, first and second slowwave propagation layers may be provided, with a first slow wavepropagation layer arranged between a first surface of the piezoelectriclayer and a first guided wave confinement structure, and with a secondslow wave propagation layer arranged between a second surface of thepiezoelectric layer and a second guided wave confinement structure. Incertain embodiments, at least one (or each) slow wave propagation layerincludes a thickness that differs from a thickness of each layer of theat least one guided wave confinement structure. In certain embodiments,multiple electrodes are arranged in at least one slow wave propagationlayer and in contact with the single crystal piezoelectric layer. Incertain embodiments, a first IDT includes a first group of electrodes ofa first polarity and a second group of electrodes of a second polarityopposing the first polarity. In certain embodiments, the second group ofelectrodes may be arranged in a plurality of recessed regions in thepiezoelectric layer and are arranged non-coplanar with the first groupof electrodes. In certain embodiments, at least one functional layer maybe arranged to at least partially cover at least some electrodes. Incertain embodiments, a first interdigital transducer (IDT) is arrangedon or in (e.g., at least partially embedded in) a first surface of apiezoelectric layer, optionally in combination with a second IDTarranged on or in a second surface of the piezoelectric layer. Incertain embodiments, multiple electrodes and a piezoelectric layer incombination embody a periodically poled transducer (PPT), with at leastone slow wave propagation layer provided between the PPT and at leastone guided wave confinement structure.

In another aspect, a MEMS guided wave device includes multipleelectrodes arranged in or on a single crystal piezoelectric layer andconfigured for transduction of a lateral acoustic wave in the singlecrystal piezoelectric layer. At least one guided wave confinementstructure arranged proximate to the single crystal piezoelectric layerconfines a laterally excited wave having a wavelength A in the singlecrystal piezoelectric layer, wherein each guided wave confinementstructure comprises a thickness of less than 5λ. The guided wave deviceincludes at least one of the following features (i) and (ii): (i) the atleast one guided wave confinement structure includes a fast wavepropagation layer, or (ii) the at least one guided wave confinementstructure includes a Bragg mirror, wherein the Bragg mirror is separatedfrom the single crystal piezoelectric layer by a slow wave propagationlayer. In certain embodiments, the guided wave device is devoid ofcontact between the electrodes and at least one (or each) guided waveconfinement structure. In certain embodiments, spacing between a guidedwave confinement structure and a single crystal piezoelectric layer maybe provided with at least one slow wave propagation layer and/or atemperature compensation layer (wherein both utilities may optionally beprovided by a single material in appropriate instances), wherein thelayer providing such spacing may embody a thickness that differs from athickness of each layer of the at least one guided wave confinementstructure. A bonded interface is preferably arranged between the singlecrystal piezoelectric layer and at least one underlying layer (such as,but not limited to, (i) a guided wave confinement structure of the atleast one guided wave confinement structure or (ii) an optionallyprovided slow wave propagation layer arranged between the single crystalpiezoelectric layer and a guided wave confinement structure of the atleast one guided wave confinement structure).

In another aspect, a single crystal piezoelectric layer of a MEMS guidedwave device includes differing first and second thickness regions, afirst group of electrodes arranged on or adjacent to the first thicknessregion and configured for transduction of a first lateral acoustic wavehaving a wavelength λ₁ in the first thickness region, and a second groupof electrodes arranged on or adjacent to the second thickness region andconfigured for transduction of a second lateral acoustic wave having awavelength λ₂ in the second thickness region, wherein λ₁ differs fromλ₂. The device further includes at least one guided wave confinementstructure configured to confine the first lateral acoustic wave in thefirst thickness region, and configured to confine the second lateralacoustic wave in the second thickness region. In certain embodiments,the at least one guided wave confinement structure includes a fast wavepropagation material. In certain embodiments, the at least one guidedwave confinement structure includes a Bragg mirror with at least onegroup of at least one low impedance layer and at least one highimpedance layer, wherein the at least one low impedance layer issequentially arranged with the at least one high impedance layer in theat least one group. In certain embodiments, the Bragg mirror isseparated from the single crystal piezoelectric layer by a temperaturecompensation layer.

In certain embodiments, the first and second groups of electrodesinclude first and second IDTs, and/or the first and second groups ofelectrodes are non-coplanar relative to one another. In certainembodiments, at least one temperature compensation layer is providedbetween the at least one guided wave confinement structure and at leasta portion of the piezoelectric layer. Optionally, a temperaturecompensation layer may include a first temperature compensation layerthickness proximate to the first thickness region of the piezoelectriclayer, and may include a second temperature compensation layer thicknessproximate to the second thickness region of the piezoelectric layer. Incertain embodiments, a temperature compensation layer may also embody aslow wave propagation material.

In certain embodiments, a MEMS guided wave device disclosed hereinfurther includes a carrier substrate having a thickness of greater than5 times the wavelength λ of a laterally excited wave confined in asingle crystal piezoelectric layer, with at least one guided waveconfinement structure arranged between the carrier substrate and thepiezoelectric layer. In certain embodiments, a MEMS guided wave deviceis solidly mounted to a carrier substrate, or portions of a MEMS guidedwave device may be suspended over a carrier substrate and separated byan intervening cavity. In other embodiments, a MEMS guided wave deviceas disclosed herein is devoid of a carrier substrate.

In another aspect, a MEMS guided wave device includes a segmented singlecrystal piezoelectric layer with multiple electrodes arranged therein orthereon and configured for transduction of a lateral acoustic wavehaving a wavelength A in the piezoelectric layer, with the multipleelectrodes including a segmented layer of first electrodes. At least oneguided wave confinement structure (preferably having a thickness of lessthan 5λ) is arranged proximate to the segmented piezoelectric layer andconfigured to confine the lateral acoustic wave in the segmentedpiezoelectric layer. Additionally, segments of the segmented singlecrystal piezoelectric layer are substantially registered (e.g.,overlapping) with segments of the segmented layer of first electrodes.In certain embodiments, a second electrode (e.g., including asubstantially continuous layer, or a discontinuous or segmented layer)is additionally provided, such as along a second surface of thepiezoelectric layer that opposes a first surface of the piezoelectriclayer in contact with the segmented layer of first electrodes. Incertain embodiments, first and second guided wave confinement structuresare provided, and gaps between segments of the segmented layer of firstelectrodes, as well as gaps between segments of the segmented singlecrystal piezoelectric layer, are filled with a slow wave propagationmaterial and/or a temperature compensation material. In certainembodiments, a layer of slow wave propagation material and/or atemperature compensation material may be provided between (i) thesegmented layer of first electrodes and (ii) at least one of the firstguided wave confinement structure or the second guided wave confinementstructure.

In another aspect, a method of fabricating a micro-electrical-mechanicalsystem (MEMS) guided wave device including a single crystalpiezoelectric material with different thickness regions is provided. Asingle crystal piezoelectric layer is locally thinned to define firstand second thickness regions that differ in thickness. The locallythinned piezoelectric layer is bonded on or over an underlying layer(e.g., at least one of (i) a fast wave propagation layer; (ii) a Braggmirror, or (iii) a substrate) to provide an internally bonded interface.Such bonding may be performed using wafer bonding techniques known inthe art. First and second groups of electrodes are defined on oradjacent to the first thickness region and the second thickness region,respectively, for transduction of a first lateral acoustic wave having afirst wavelength λ₁ in the first thickness region, and for transductionof a second lateral acoustic wave having a second wavelength λ₂ in thesecond thickness region. In certain embodiments, one or more surfaces ofthe piezoelectric layer are planarized prior to bonding (e.g., as abonding preparation step), and/or planarized after bonding (e.g., toadjust thickness of the piezoelectric layer). In certain embodiments, atemperature compensation layer may be provided below the piezoelectriclayer, with the temperature compensation layer optionally including afirst temperature compensation layer thickness region and a secondtemperature compensation layer thickness region that differ from oneanother. In certain embodiments, a temperature compensation material isdeposited on or over a surface of at least one of the first thicknessregion or the second thickness region.

In another aspect, any of the foregoing aspects, and/or various separateaspects and features as described herein, may be combined for additionaladvantage. Any of the various features and elements as disclosed hereinmay be combined with one or more other disclosed features and elementsunless indicated to the contrary herein.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description in association with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thisspecification illustrate several aspects of the invention, and togetherwith the description serve to explain the principles of the invention.

FIG. 1 is a perspective view of a MEMS guided wave device including anIDT and two reflector gratings arranged over a piezoelectric layer, anoptional slow wave propagation layer, and a guided wave confinementstructure according to one embodiment of the present disclosure.

FIG. 2 is a perspective view of a MEMS guided wave device including anIDT arranged over portions of a piezoelectric layer and a guided waveconfinement structure suspended between anchors according to oneembodiment of the present disclosure.

FIG. 3 is a perspective view of a MEMS guided wave device including anIDT arranged over portions of a piezoelectric layer and a guided waveconfinement structure suspended by narrow mechanical supports betweenanchors according to one embodiment of the present disclosure.

FIG. 4 is a side cross-sectional view of a MEMS guided wave deviceincluding top side electrodes in the form of an IDT arranged over apiezoelectric layer, a slow wave propagation layer, a fast wavepropagation layer serving as a one-sided guided wave confinementstructure, and a carrier substrate according to one embodiment of thepresent disclosure.

FIG. 5 is a side cross-sectional view of a MEMS guided wave deviceincluding top side electrodes in the form of an IDT arranged over apiezoelectric layer, a slow wave propagation layer, a Bragg mirrorserving as a one-sided guided wave confinement structure, and a carriersubstrate according to one embodiment of the present disclosure.

FIG. 6 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT arranged on only one face ofa piezoelectric layer, with slow wave propagation layers sandwiching theelectrodes and the piezoelectric layer, and two-sided guided waveconfinement provided by first and second fast wave propagation layersaccording to one embodiment of the present disclosure.

FIG. 7 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT arranged on one face of apiezoelectric layer, with slow wave propagation layers sandwiching theelectrodes and the piezoelectric layer, and two-sided guided waveconfinement provided by first and second Bragg mirrors according to oneembodiment of the present disclosure.

FIG. 8 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of first and second IDTs arranged onfirst and second faces of a piezoelectric layer, respectively, with slowwave propagation layers sandwiching the electrodes and the piezoelectriclayer, and two-sided guided wave confinement provided by first andsecond fast wave propagation layers according to one embodiment of thepresent disclosure.

FIG. 9 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of first and second IDTs arranged onfirst and second faces of a piezoelectric layer, respectively, with slowwave propagation layers sandwiching the electrodes and the piezoelectriclayer, and two-sided guided wave confinement provided by first andsecond Bragg mirrors according to one embodiment of the presentdisclosure.

FIG. 10 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT arranged on only one face ofa piezoelectric layer within a slow wave propagation layer, and withone-sided guided wave confinement provided by a fast wave propagationlayer according to one embodiment of the present disclosure.

FIG. 11 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT arranged on only one face ofa piezoelectric layer within a slow wave propagation layer, and withone-sided guided wave confinement provided by a Bragg mirror accordingto one embodiment of the present disclosure.

FIG. 12 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in and extendingthrough the entire thickness of a piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, and two-sided guided wave confinement provided by first andsecond fast wave propagation layers according to one embodiment of thepresent disclosure.

FIG. 13 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in and extendingthrough the entire thickness of a piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, and two-sided guided wave confinement provided by first andsecond Bragg mirrors according to one embodiment of the presentdisclosure.

FIG. 14 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in a single face ofa piezoelectric layer, with a slow wave propagation layer contacting theelectrodes and the piezoelectric layer, and one-sided guided waveconfinement provided by a fast wave propagation layer according to oneembodiment of the present disclosure.

FIG. 15 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in a single face ofa piezoelectric layer, with a slow wave propagation layer contacting theelectrodes and the piezoelectric layer, and one-sided guided waveconfinement provided by a Bragg mirror according to one embodiment ofthe present disclosure.

FIG. 16 is a side cross-sectional view of a MEMS guided wave deviceincluding a periodically poled transducer (PPT) having electrode layerssandwiching a piezoelectric layer with alternating polarity regions, andfurther including a slow wave propagation layer, a fast wave propagationlayer serving as a one-sided guided wave confinement structure, and acarrier substrate according to one embodiment of the present disclosure.

FIG. 17 is a side cross-sectional view of a MEMS guided wave deviceincluding a PPT having electrode layers sandwiching a piezoelectriclayer with alternating polarity regions, and further including a slowwave propagation layer, a Bragg mirror serving as a one-sided guidedwave confinement structure, and a carrier substrate according to oneembodiment of the present disclosure.

FIG. 18 is a side cross-sectional view of a MEMS guided wave deviceincluding a PPT having electrode layers sandwiching a piezoelectriclayer with alternating polarity regions, and further including slow wavepropagation layers sandwiching the PPT, first and second fast wavepropagation layers serving as a two-sided guided wave confinementstructure, and a carrier substrate according to one embodiment of thepresent disclosure.

FIG. 19 is a side cross-sectional view of a MEMS guided wave deviceincluding a PPT having electrode layers sandwiching a piezoelectriclayer with alternating polarity regions, and further including slow wavepropagation layers sandwiching the PPT, first and second Bragg mirrorsserving as a two-sided guided wave confinement structure, and a carriersubstrate according to one embodiment of the present disclosure.

FIG. 20 is a side cross-sectional view of a MEMS guided wave deviceincluding a segmented piezoelectric layer arranged between a continuouselectrode and a segmented electrode registered with the piezoelectriclayer, with the device including a slow wave propagation layer, a fastwave propagation layer serving as a one-sided wave confinementstructure, and a carrier substrate according to one embodiment of thepresent disclosure.

FIG. 21 is a side cross-sectional view of a MEMS guided wave deviceincluding a segmented piezoelectric layer arranged between a continuouselectrode and a segmented electrode registered with the piezoelectriclayer, with the device including a slow wave propagation layer,including a Bragg mirror serving as a one-sided wave confinementstructure, and including a carrier substrate according to one embodimentof the present disclosure.

FIG. 22 is a side cross-sectional view of a MEMS guided wave deviceincluding a segmented piezoelectric layer arranged between a continuouselectrode and a segmented electrode registered with the piezoelectriclayer, with the device including slow wave propagation material (i)between the segmented piezoelectric layer and the segmented electrode,and (ii) sandwiching the electrodes, including fast wave propagationlayers serving as a two-sided wave confinement structure, and includinga carrier substrate according to one embodiment of the presentdisclosure.

FIG. 23 is a side cross-sectional view of a MEMS guided wave deviceincluding a segmented piezoelectric layer arranged between a continuouselectrode and a segmented electrode registered with the piezoelectriclayer, with the device including slow wave propagation material (i)between the segmented piezoelectric layer and the segmented electrode,and (ii) sandwiching the electrodes, including Bragg mirrors serving asa two-sided wave confinement structure, and including a carriersubstrate according to one embodiment of the present disclosure.

FIG. 24 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in and extendingthrough the entire thickness of a piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, with two-sided guided wave confinement provided by a fast wavepropagation layer arranged below the piezoelectric layer and a Braggmirror arranged above the piezoelectric layer, and with a carriersubstrate according to one embodiment of the present disclosure.

FIG. 25 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT embedded in and extendingthrough the entire thickness of a piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, with two-sided guided wave confinement provided by a Bragg mirrorarranged below the piezoelectric layer and a fast wave propagation layerarranged above the piezoelectric layer, and with a carrier substrateaccording to one embodiment of the present disclosure.

FIG. 26 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT with certain electrodesdeposited in recesses defined in a piezoelectric layer and otherelectrodes deposited atop the piezoelectric layer, with the devicefurther including a slow wave propagation layer, one-sided guided waveconfinement provided by a fast wave propagation layer, and a carriersubstrate according to one embodiment of the present disclosure.

FIG. 27 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT with certain electrodesdeposited in recesses defined in a piezoelectric layer and otherelectrodes deposited atop the piezoelectric layer, with the devicefurther including a slow wave propagation layer, one-sided guided waveconfinement provided by a Bragg mirror, and a carrier substrateaccording to one embodiment of the present disclosure.

FIG. 28 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT with certain electrodesdeposited in recesses defined in a piezoelectric layer and otherelectrodes deposited atop the piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, with two-sided guided wave confinement provided by fast wavepropagation layers arranged above and below the slow wave propagationlayers, and with a carrier substrate according to one embodiment of thepresent disclosure.

FIG. 29 is a side cross-sectional view of a MEMS guided wave deviceincluding electrodes in the form of an IDT with certain electrodesdeposited in recesses defined in a piezoelectric layer and otherelectrodes deposited atop the piezoelectric layer, with slow wavepropagation layers sandwiching the electrodes and the piezoelectriclayer, with two-sided guided wave confinement provided by Bragg mirrorsarranged above and below the slow wave propagation layers, and with acarrier substrate according to one embodiment of the present disclosure.

FIG. 30 is a side cross-sectional view of subassemblies of a MEMS guidedwave device during fabrication, following local thinning of apiezoelectric layer to define first and second thickness regions with athinner region filled with a temperature compensation material, andprior to wafer bonding of the piezoelectric layer to a subassemblyincluding a temperature compensation layer arranged over a fast wavepropagation layer and a carrier substrate according to one embodiment ofthe present disclosure.

FIG. 31 is a side cross-sectional view of a MEMS guided wave deviceproduced with the subassemblies illustrated in FIG. 30 , following waferbonding, planarization/thinning of an outer surface of the piezoelectriclayer, and deposition of substantially coplanar first and second groupsof electrodes over first and second thickness regions, respectively, ofthe piezoelectric layer, according to one embodiment of the presentdisclosure.

FIG. 32 is a side cross-sectional view of subassemblies of a MEMS guidedwave device during fabrication, following local thinning of apiezoelectric layer to define first and second thickness regions with athinner region filled with a temperature compensation material, andprior to wafer bonding of the piezoelectric layer to a subassemblyincluding a temperature compensation layer arranged over a Bragg mirrorand a carrier substrate according to one embodiment of the presentdisclosure.

FIG. 33 is a side cross-sectional view of a MEMS guided wave deviceproduced with the subassemblies illustrated in FIG. 32 , following waferbonding, planarization/thinning of an outer surface of the piezoelectriclayer, and deposition of substantially coplanar first and second groupsof electrodes over first and second thickness regions, respectively, ofthe piezoelectric layer, according to one embodiment of the presentdisclosure

FIG. 34 is a side cross-sectional view of a MEMS guided wave deviceincluding substantially coplanar first and second groups of electrodesarranged over first and second thickness regions, respectively, of apiezoelectric layer, which is arranged over a temperature compensationlayer having first and second temperature compensation layer thicknessesunderlying the first and second thickness regions of the piezoelectriclayer, respectively, which is arranged over a fast wave propagationlayer having first and second fast wave propagation layer thicknessesunderlying the first and second temperature compensation layer thicknessregions and providing guided wave confinement utility, and which isarranged over a carrier substrate according to one embodiment of thepresent disclosure.

FIG. 35 is a side cross-sectional view of a MEMS guided wave deviceincluding non-coplanar first and second groups of electrodes arrangedover non-coplanar upper surfaces bounding first and second thicknessregions, respectively, of a piezoelectric layer, which is arranged overa temperature compensation layer, a fast wave propagation layerproviding one-sided guided wave confinement utility, and a carriersubstrate according to one embodiment of the present disclosure.

FIG. 36 is a side cross-sectional view of a MEMS guided wave deviceincluding non-coplanar first and second groups of electrodes arrangedover non-coplanar upper surfaces bounding first and second thicknessregions, respectively, of a piezoelectric layer, which is arranged overa temperature compensation layer, a Bragg mirror providing one-sidedguided wave confinement utility, and a carrier substrate according toone embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the terms “proximate”and “adjacent” as applied to a specified layer or element refers to astate of being close or near to an other layer or element, and encompassthe possible presence of one or more intervening layers or elementswithout necessarily requiring the specified layer or element to bedirectly on or directly in contact with the other layer or elementunless specified to the contrary herein.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates in one aspect to amicro-electrical-mechanical system (MEMS) guided wave device thatutilizes a single crystal piezoelectric layer and at least one guidedwave confinement structure (e.g., a fast wave propagation layer or aBragg mirror) configured to confine a laterally excited wave in thesingle crystal piezoelectric layer. Such confinement may significantlyreduce or avoid dissipation of acoustic radiation in a substrate orother carrier structure. The MEMS guided wave device may have dominantlateral vibrations. The single crystal piezoelectric layer may includelithium tantalate or lithium niobate, and may provide vibratingstructures with precise sizes and shapes, which may provide highaccuracy, and (in at least certain embodiments) may enable fabricationof multiple resonators having different resonant frequencies on a singlesubstrate.

Vibrating structures of preferred MEMS guided wave devices describedherein are formed of single crystal piezoelectric material and usemechanically efficient MEMS construction. Such vibrating structures maybe high-Q, low loss, stable, at a low temperature coefficient offrequency, have a high electromechanical coupling efficient, have highrepeatability, and have a low motional impedance. In certainembodiments, a nonstandard (e.g., offcut) crystalline orientation of thesingle crystal piezoelectric material may be used to provide specificvibrational characteristics, such as a low temperature coefficient offrequency, a high electromechanical coupling coefficient, or both. Sinceit is extremely difficult to grow single crystal piezoelectric material(e.g., via epitaxy) over non-lattice-matched materials, in preferredembodiments, single crystal piezoelectric materials are pre-fabricated(e.g., by growth of a boule followed by formation of thin wafers),surface finished (e.g., via chemical mechanical planarization (CMP) andpolishing to provide near-atomic flatness), and bonded to one or moreunderlying layers—such as may include a guided wave confinementstructure that is optionally overlaid with a layer providing slow wavepropagation and/or temperature compensation utility, and that isoptionally supported by a carrier substrate. Any suitable wafer bondingtechnique known in the art may be used, such as may rely on van derWaals bonds, hydrogen bonds, covalent bond, and/or mechanicalinterlocking. In certain embodiments, direct bonding may be used. Incertain embodiments, bonding may include one or more surface activationsteps (e.g., plasma treatment, chemical treatment, and/or othertreatment methods) followed by application of heat and/or pressure,optionally followed by one or more annealing steps. Such bonding resultsin formation of a bonded interface between the piezoelectric layer andat least one underlying layer. In certain embodiments, the bondedinterface may include at least one intervening layer arranged on atleast a portion of (or the entirety of) a surface of the piezoelectriclayer. Suitable electrodes may be defined in or on the piezoelectriclayer for transduction of at least one lateral acoustic wave therein.One or more additional layers (e.g., one or more layers providingadditional (two-sided) guided wave confinement utility, and one or morelayers providing slow wave propagation utility) may be further providedover the piezoelectric layer.

In certain embodiments, a composite including a single crystalpiezoelectric layer, at least one guided wave confinement structure, andelectrodes (optionally in combination with one or more additional layersproviding slow wave propagation and/or temperature compensation utilityas disclosed herein) is solidly mounted to a carrier substrate. In otherembodiments, at least a portion of such a composite may be suspendedabove a carrier substrate with a gap arranged therebetween. According topreferred embodiments, no portion of a piezoelectric layer is suspendedon its own in the absence of at least one additional layer as describedherein. In this regard, the present disclosure relates to plate-type orquasi-plate-type guided wave devices suitable for lateral wavepropagation, as opposed to membrane-type devices. In certainembodiments, devices described herein may be used for propagation ofquasi-shear horizontal waves, quasi-longitudinal waves, and/orthickness-extensional (FBAR-type) waves.

The terms “fast wave propagation material” or “fast wave propagationlayer” refers to a material or layer in which an acoustic wave ofinterest travels more quickly than in a proximate piezoelectric layer inwhich the acoustic wave is transduced. Similarly, the terms “slow wavepropagation material” or “slow wave propagation layer” refers to amaterial or layer in which an acoustic wave of interest travels moreslowly than in a proximate piezoelectric layer in which the acousticwave is transduced. Examples of fast wave propagation materials that maybe used according to certain embodiments include (but are not limitedto) diamond, sapphire, aluminum nitride, silicon carbide, boron nitride,and silicon. An example of a slow wave propagation material that may beused according to certain embodiments includes (but is not limited to)silicon dioxide.

Certain embodiments disclosed herein utilize acoustic Bragg mirrors(also known as Bragg reflectors). A Bragg mirror includes at least onegroup of at least one low impedance layer (e.g., silicon dioxide) and atleast one high impedance layer (e.g., tungsten or hafnium dioxide),wherein the at least one low impedance layer is sequentially arrangedwith the at least one high impedance layer in the at least one group.The number of groups of alternating impedance layers used in a Braggmirror depends on the total reflection coefficient required.

Single crystal piezoelectric layers as disclosed herein preferablyinclude a thickness of no greater than 2 times the wavelength A (morepreferably no greater than 1 times the wavelength, or no greater than0.5 times the wavelength) of a lateral acoustic wave transduced in thepiezoelectric layer. As disclosed herein, a guided wave confinementstructure arranged proximate to a single crystal piezoelectric layerpreferably includes a thickness of less than 5λ (e.g., within a range of1λ to 5λ). (Within a Bragg mirror, each layer may include a thickness onthe order of roughly 0.25λ to 0.5λ.) If provided, any optional slow wavepropagation layers may have individual thicknesses in a range of up toabout 1λ, and may preferably be less than about 0.5λ or less than about0.25λ. In certain embodiments, each slow wave propagation layer may havea thickness of less than a thickness of an adjacent single crystalpiezoelectric layer. This preferred guided wave confinement structurethickness is to be contrasted with a carrier substrate that may beprovided according to certain embodiments, wherein such a carriersubstrate preferably includes a thickness of greater than 5λ (or fivetimes the greatest wavelength in embodiments in which multipleresonators of different frequencies are provided in a single guided wavedevice). In alternative embodiments applicable to any structuresdescribed herein, however, a fast layer may have a thickness of greaterthan 5λ, and may embody a substrate of any necessary or desiredthickness. When provided, at least one functional layer (e.g., providingslow wave propagation and/or thermal compensation utility) may desirablyhave a thickness of no greater than 2λ, and/or a thickness that differsfrom a thickness of each layer of the at least one guided waveconfinement structure.

Although lithium niobate and lithium tantalate are particularlypreferred piezoelectric materials, in certain embodiments any suitablepiezoelectric materials may be used, such as quartz, a piezoceramic, ora deposited piezoelectric material (such as aluminum nitride or zincoxide).

Guided wave devices as disclosed herein may incorporate variouscombinations of electrode configurations and guided wave confinementstructure configurations as illustrated in the drawings and describedherein. In certain embodiments, electrodes are arranged symmetricallyrelative to a center thickness of a piezoelectric layer (e.g., arrangedboth above and below the piezoelectric layer, or embedded along a planeequidistant between upper and lower surfaces of the piezoelectric layer)for symmetric guided wave excitation. In other embodiments, electrodesare asymmetrically arranged relative to a center thickness of apiezoelectric layer for asymmetric guided wave excitation. In certainembodiments, MEMS guided wave devices as disclosed herein may employsingle-sided confinement, in which at least one confinement structure isprovided adjacent to a first surface of a single crystal piezoelectriclayer, and in which a solid/air interface is provided adjacent to asecond opposing surface of the single crystal piezoelectric layer.Single sided confinement may be employed in combination with symmetricor asymmetric excitation. In other embodiments, MEMS guided wave devicesas disclosed herein may employ double- or two-sided confinement, inwhich first and second confinement structures are provided proximate tofirst and second opposing surfaces, respectively, of a single crystalpiezoelectric layer. In certain embodiments, a first guided waveconfinement structure is proximate to a first surface of a piezoelectriclayer, and a second guided wave confinement structure is proximate to asecond surface of the piezoelectric layer. Two-sided confinement may beemployed in combination with symmetric or asymmetric excitation.Electrode configurations that may be employed according to certainembodiments include, but are not limited to, single layer coplanarinterdigital transducers (IDTs) alone, multiple layer coplanar IDTsalone, IDTs in combination with electrodes along a second surface of apiezoelectric layer (e.g., continuous layer electrodes useable asfloating or shorting electrodes, or segmented or discontinuouselectrodes), IDTs at least partially embedded in piezoelectric layers,non-coplanar IDTs, IDTs registered with single crystal piezoelectriclayer segments, and periodically poled transducers (PPTs). In certainembodiments, electrodes may be partially embedded in, and/or gapsbetween various electrodes may be filled in whole or in part with, (i)piezoelectric material, or (ii) slow wave propagation material and/ortemperature compensation material.

For each embodiment involving two-sided confinement disclosed herein,alternative embodiments omitting the second (top) side confinementstructure are specifically contemplated.

Material that has the potential to become piezoelectric may have acrystalline structure with randomly oriented dipoles. The materialbecomes piezoelectric by substantially aligning the dipoles to formdomains having a substantially uniform dipole orientation, which may becreated by poling. Poling may include applying a strong poling electricfield to a region of the material to substantially force the dipolesinto alignment. When the electric field is removed, much of thealignment remains, thereby providing the piezoelectric properties of thepoled material, which is called piezoelectric material. In certaininstances, a first set of domains have a nominal domain orientation, anda second set of domains may have an inverted domain (e.g., translatedabout 180 degrees from the nominal domain). Nominal and inverted domainsmay be alternately arranged within a periodically poled piezoelectriclayer. When such a layer is arranged between first and second electrodelayers, the result is a periodically poled transducer.

An interdigital transducer includes electrodes with a first conductingsection and a second conducting section that are inter-digitallydispersed in or on a surface or layer. IDTs are well known in the art.

In certain embodiments, at least one functional layer is arranged to atleast partially cover at least some electrodes of a plurality ofelectrodes. In certain embodiments, at least one functional layer coversone group of electrodes, but does not cover another group of electrodes.A functional layer may modify velocity of a transduced acoustic waveand/or alter temperature compensation properties of a MEMS guided wavedevice. In certain embodiments, at least one functional layer includes atemperature compensation material or a slow wave propagation material.

Although various embodiments disclosed herein include single resonators,it is to be appreciated that any suitable combinations of single ormultiple resonators and/or reflector gratings in series and/or inparallel (such as may be embodied in one or more filters) may beprovided in a single MEMS guided wave device. In certain embodiments,multiple resonators and/or filters arranged for transduction of acousticwaves of different wavelengths may be provided in a single MEMS guidedwave device.

FIG. 1 illustrates a MEMS guided wave device 10 according to oneembodiment of the present disclosure. The device 10 includes a singlecrystal piezoelectric layer 12 (such as lithium niobate or lithiumtantalate) on which an IDT 18 and two reflector gratings 20 areprovided. The single crystal piezoelectric layer 12 is arranged over aguided wave confinement structure 16, with an optional slow wavepropagation layer 14 arranged therebetween. Although not illustrated, anoptional carrier substrate may be provided below the guided waveconfinement structure 16 in certain embodiments. Both the IDT 18 and thereflector gratings 20 include a number of fingers 24 that are connectedto respective bus bars 22. For the reflector gratings 20, all fingers 24connect to each bus bar 22. For the IDT 18, alternating fingers 24connect to different bus bars 22, as depicted. Notably, actual reflectorgratings 20 and IDTs 18 generally include much larger numbers of fingers24 than illustrated. The number of fingers 24 has been reduced in FIG. 1and various accompanying drawings to promote drawing clarity and forease of explaining the disclosure.

The fingers 24 are parallel to one another and aligned in an acousticregion that encompasses the area in which the reflector gratings 20 andthe IDT 18 reside. The wave or waves generated when the IDT 18 isexcited with electrical signals essentially reside in this acousticregion. Acoustic waves essentially travel perpendicular to the length ofthe fingers 24. The guided wave confinement structure 16, which mayinclude a fast wave propagation layer or a Bragg mirror, serves toconfine the wave or waves in the single crystal piezoelectric layer 12.

The operating frequency of the MEMS guided wave device 10 is a functionof the pitch (P) representing the spacing between fingers 24 of the IDT18, wherein the wavelength λ equals two times the pitch P. Lateral modedevices also have preferred thickness ranges for the piezoelectric layer12 for efficient excitation of lateral waves.

To manufacture the MEMS guided wave device 10, a single crystalpiezoelectric wafer may be prefabricated, and separately the slow wavepropagation layer 14 may be deposited on the guided wave confinementstructure 16 (which may optionally be supported by a carrier substrate).Adjacent surfaces of the piezoelectric wafer and the slow wavepropagation layer 14 are planarized and polished, and then attached toone another via a conventional direct bonding (e.g., wafer bonding)process or other process. One or more bonding promoting layers mayoptionally be arranged between the respective layers to be bonded.Following bonding, the exposed upper surface of the piezoelectric layer12 is ground (optionally also planarized) to a desired thickness, andthe reflector gratings 20 and the IDT 18 are deposited thereon.

In certain embodiments, the slow wave propagation layer 14 may providethermal compensation utility. The materials used to form the singlecrystal piezoelectric wafer typically have different thermalcoefficients of expansion (TCE) relative to the TCE of materials of theguided wave confinement structure 16. Once the guided wave confinementstructure 16 is created, the piezoelectric layer 12 and the slow wavepropagation layer 14 tend to expand and contract in a similar manner astemperature changes. As such, the expansion and contraction forcesapplied to the guided wave confinement structure 16 by the piezoelectriclayer 12 due to temperature changes are substantially countered byopposing forces applied by the intermediately arranged slow wavepropagation layer 14. As a result, the composite structure including theintermediately arranged slow wave propagation layer 14 resists bendingor warping as temperature changes, thereby reducing expansion andcontraction of the piezoelectric layer 12, and reducing the effectiveTCE of the piezoelectric layer 12.

Since providing the slow wave propagation layer 14 between thepiezoelectric layer 12 and the guided wave confinement structure 16reduces the effective TCE of the piezoelectric material, the amount ofexpansion and contraction along the surface of the piezoelectric layer12 as temperature changes is reduced. Therefore, the change in spacing,or pitch, between fingers 24 of the IDT 18 and the reflector gratings 20as temperature changes is reduced, thereby reducing the effectivethermal coefficient of frequency (TCF) of the piezoelectric layer 12 toimprove overall frequency response of the IDT 18 and the reflectorgratings 20 with changes in temperature.

A MEMS guided wave structure 10 as illustrated in FIG. 1 may be solidlymounted to a carrier substrate, or alternatively at least a portion ofsuch a structure may be suspended above a carrier substrate with a gaparranged therebetween.

FIG. 2 illustrates a MEMS guided wave device 30 including an IDT 18arranged over portions of a single crystal piezoelectric layer 12 and aguided wave confinement structure 16 in combination that are elevatedabove a substrate 28 and suspended between anchors 32, 34 according toone embodiment of the present disclosure. A conductive layer 36 isarranged on the piezoelectric layer 12 to form the IDT 18 includingelectrodes, providing a first conductive section 38 and a secondconductive section 40. The single crystal piezoelectric layer 12 isarranged over the guided wave confinement structure 16, supported alongperipheral portions thereof from below by an insulating layer 26 locatedover the substrate 28. A central portion of the insulating layer 26 maybe removed by etching (e.g., using vias or other openings (not shown)defined through the substrate 28 or the piezoelectric layer 12 andguided wave confinement structure 16), leaving anchors 32, 34 laterallybounding a central cavity. The central portions of the guided waveconfinement structure 16 and the overlying piezoelectric layer 12 (bothin substantially continuous form), together with the IDT 18, aresuspended between the anchors 32, 34. Although not shown, in certainembodiments the MEMS guided wave device 30 may further include a slowwave propagation layer between the single crystal piezoelectric layer 12and the guided wave confinement structure 16, which may providetemperature compensation utility.

FIG. 3 illustrates a MEMS guided wave device 42 including an IDT 18arranged over portions of a single crystal piezoelectric layer 12 and aguided wave confinement structure 16 in combination that are elevatedabove a substrate 28 and suspended by narrowed mechanical supports 44,46 between anchors 32, 34 according to one embodiment of the presentdisclosure. A conductive layer 36 is arranged on the piezoelectric layer12 to form the IDT 18, including electrodes providing a first conductivesection 38 and a second conductive section 40. The single crystalpiezoelectric layer 12 is arranged over the guided wave confinementstructure 16, supported along peripheral portions thereof from below byan insulating layer 26 located over the substrate 28. Portions of theinsulating layer 26 may be removed by etching, leaving anchors 32, 34laterally bounding a central cavity. Additionally, portions of thesingle crystal piezoelectric layer 12 and the guided wave confinementstructure 16 are removed to leave only the narrowed mechanical supports44, 46 to support central portions of the piezoelectric layer 12 and theguided wave confinement structure 16 (as well as the IDT 18) between theanchors 32, 34. Although not shown, in certain embodiments the MEMSguided wave device 42 may further include a slow wave propagation layerbetween the single crystal piezoelectric layer 12 and the guided waveconfinement structure 16, which may provide temperature compensationutility.

Additional MEMS guided wave devices including further electrodeconfigurations and guided wave confinement structure configurations areillustrated in the following figures. Although the following figuresillustrate single crystal piezoelectric layers and guided waveconfinement structures appearing to be solidly mounted to carriersubstrates, it is to be appreciated that in each instance theillustrated single crystal piezoelectric layers and guided waveconfinement structures (together with accompanying electrodes) may bedevoid of a substrate or suspended above a carrier substrate (such asshown in FIGS. 2-3 ).

FIG. 4 illustrates a MEMS guided wave device including alternating topside electrodes 38, 40 in the form of an IDT arranged over a singlecrystal piezoelectric layer 12, an optional slow wave propagation layer14, a fast wave propagation layer serving as a one-sided guided waveconfinement structure 16, and an optional carrier substrate 28 accordingto one embodiment of the present disclosure. The electrodes 38, 40 aredisposed solely on a top surface of the piezoelectric layer 12 and aretherefore asymmetrically arranged relative to a center of thepiezoelectric layer 12. The guided wave confinement structure 16 andoptional slow wave propagation layer 14 may be deposited over thesubstrate 28. The MEMS guided wave device may be formed by bonding aprefabricated single crystal piezoelectric wafer over the guided waveconfinement structure 16 (e.g., with the slow wave propagation layer 14therebetween), processing an exposed surface of the piezoelectric waferto a desired thickness to yield the piezoelectric layer 12, anddepositing the electrodes 38, 40 thereon.

FIG. 5 illustrates a MEMS guided wave device including alternating topside electrodes 38, 40 in the form of an IDT arranged over a singlecrystal piezoelectric layer 12, an optional slow wave propagation layer14, a Bragg mirror serving as a one-sided guided wave confinementstructure 16, and an optional carrier substrate 28 according to oneembodiment of the present disclosure. The device of FIG. 5 may befabricated in a manner similar to the device of FIG. 4 .

FIG. 6 illustrates a MEMS guided wave device including alternatingelectrodes 38, 40 in the form of an IDT arranged over one surface of asingle crystal piezoelectric layer 12 according to one embodiment of thepresent disclosure. Optional slow wave propagation layers 14A, 14Bsandwich the electrodes 38, 40 and the piezoelectric layer 12. A portionof the upper slow wave propagation layer 14B is arranged to at leastpartially (and preferably completely) fill gaps 48 between theelectrodes 38, 40. Two-sided guided wave confinement is provided byfirst and second fast wave propagation layers serving as first andsecond guided wave confinement structures 16A, 16B that sandwich theslow wave propagation layers 14A, 14B. An optional substrate 28 isdisposed under the first guided wave confinement structure 16A. Theelectrodes 38, 40 are disposed solely on one surface of thepiezoelectric layer 12 and are therefore asymmetrically arrangedrelative to a center of the piezoelectric layer 12. A lower portion ofthe device of FIG. 6 may be fabricated in a manner similar to the MEMSguided wave device of FIG. 4 . After formation of the electrodes 38, 40,the optional second slow wave propagation layer 14B is deposited overthe piezoelectric layer 12 and the electrodes 38, 40, and the secondguided wave confinement structure 16B is deposited over the second slowwave propagation layer 14B.

FIG. 7 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 6 , except that first and second Bragg mirrors areused instead of first and second fast wave propagation layers 16A, 16B.Fabrication of the device of FIG. 7 is substantially similar tofabrication of the device of FIG. 6 . Although FIGS. 6 and 7 depict MEMSguided wave devices with two-sided confinement, in alternativeembodiments the second (top) side confinement structure may be omitted.

FIG. 8 illustrates a MEMS guided wave device including alternatingelectrodes 38A, 40A in the form of a first IDT arranged on the lowersurface of a single crystal piezoelectric layer 12 and alternating topside electrodes 38B, 40B in the form of a second IDT arranged on theupper surface of the piezoelectric layer 12, according to one embodimentof the present disclosure. Optional slow wave propagation layers 14A,14B sandwich the electrodes 38A, 40A, 38B, 40B and the piezoelectriclayer 12, with portions of the slow wave propagation layers 14A, 14Bbeing arranged to at least partially (and preferably completely) fillgaps 48A, 48B between the electrodes 38A, 40A, 38B, 40B. The electrodes38A, 40A, 38B, 40B are symmetrically arranged relative to a center ofthe piezoelectric layer 12. Two-sided guided wave confinement isprovided by first and second fast wave propagation layers serving asfirst and second guided wave confinement structures 16A, 16B thatsandwich the slow wave propagation layers 14A, 14B. An optionalsubstrate 28 is disposed under the first guided wave confinementstructure 16A. To fabricate the MEMS guided wave device of FIG. 8 , alower subassembly including the substrate 28, the first guided waveconfinement structure 16A, and the first slow wave propagation layer 14Ais produced. Recesses may be defined (e.g., via etching) in an uppersurface of the first slow wave propagation layer 14A, and metal may bedeposited in the recesses to form the lower electrodes 38A, 40A.Following planarization and polishing of the first slow wave propagationlayer 14A and lower electrodes 38A, 40A, a prefabricated single crystalpiezoelectric wafer may be directly bonded to the lower subassembly andprocessed (e.g., via grinding and polishing) to a desired thickness toform the piezoelectric layer 12. Thereafter, the upper electrodes 38B,40B may be deposited on the piezoelectric layer 12, followed bydeposition of the second slow wave propagation layer 14B and the secondguided wave confinement structure 16B.

FIG. 9 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 8 , except that first and second Bragg mirrors areused instead of first and second fast wave propagation layers as thefirst and second guided wave confinement structures 16A, 16B.Fabrication of the device of FIG. 9 is substantially similar tofabrication of the device of FIG. 8 . Although FIGS. 8 and 9 depict MEMSguided wave devices with two-sided confinement, in alternativeembodiments the second (top) side confinement structure 16B may beomitted, optionally in combination with omission of the second slow wavepropagation layer 14B.

Although FIGS. 8 and 9 illustrate two IDTs in phase (wherein electrodesof corresponding types (e.g., 38A, 38B or 40A, 40B) are aligned with oneanother on opposite faces of the piezoelectric layer 12, such thatpositive overlies positive and negative overlies negative), in alternateembodiments, IDTs may be arranged out of phase, with electrodes ofopposing types (e.g., 38A, 40B or 40A, 38B) being aligned with oneanother on opposite faces of the piezoelectric layer 12, such thatpositive overlies negative, and vice-versa. This modification may beapplied to any embodiments disclosed herein in which multiple IDTs arearranged on or adjacent to opposing faces of a piezoelectric layer.

FIG. 10 illustrates a MEMS guided wave device including alternatingelectrodes 38, 40 in the form of an IDT arranged on the lower surface ofa single crystal piezoelectric layer 12 according to one embodiment ofthe present disclosure. An optional slow wave propagation layer 14, afast wave propagation layer serving as a one-sided guided waveconfinement structure 16, and an optional carrier substrate 28 arearranged under the electrodes 38, 40 and the piezoelectric layer 12. Theelectrodes 38, 40 are disposed solely on a bottom surface of thepiezoelectric layer 12 and are therefore asymmetrically arrangedrelative to a center of the piezoelectric layer 12. The guided waveconfinement structure 16 and optional slow wave propagation layer 14 maybe deposited over the substrate 28. A portion of the slow wavepropagation layer 14 is arranged to at least partially (and preferablycompletely) fill gaps 48 between the electrodes 38, 40. To fabricate theMEMS guided wave device of FIG. 10 , a lower subassembly including thesubstrate 28, the guided wave confinement structure 16, and the slowwave propagation layer 14 is produced. Recesses may be defined (e.g.,via etching) in an upper surface of the slow wave propagation layer 14,and metal may be deposited in the recesses to form the electrodes 38,40. Following planarization and polishing of the slow wave propagationlayer 14 and the electrodes 38, 40, a prefabricated single crystalpiezoelectric wafer may be directly bonded to the lower subassembly andprocessed (e.g., via grinding and polishing) to a desired thickness toform the piezoelectric layer 12. Alternatively, the electrodes 38, 40may be patterned on the single crystal piezoelectric wafer, the slowwave propagation layer 14 may be deposited over the electrodes, andafter planarizing the slow wave propagation layer 14 is may be bonded tothe substrate 28.

FIG. 11 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 10 , except that a Bragg mirror is used instead of afast wave propagation layer as a guided wave confinement structure 16.Fabrication of the device of FIG. 11 is substantially similar tofabrication of the device of FIG. 10 .

FIG. 12 illustrates a MEMS guided wave device including electrodes 38,40 in the form of an IDT embedded in and extending through the entirethickness of a piezoelectric layer 12, with slow wave propagation layers14A, 14B sandwiching the electrodes 38, 40 and the piezoelectric layer12. Two sided guided wave confinement is provided by first and secondfast wave propagation layers serving as first and second guided waveconfinement structures 16A, 16B that sandwich the slow wave propagationlayers 14A, 14B. An optional substrate 28 is disposed under the firstguided wave confinement structure 16A. The electrodes 38, 40 aresymmetrically arranged relative to a center thickness of thepiezoelectric layer 12. To fabricate the MEMS guided wave device of FIG.12 , a lower subassembly including the substrate 28, the first guidedwave confinement structure 16A, and the first slow wave propagationlayer 14A is produced. Following planarization and polishing of thefirst slow wave propagation layer 14A, a prefabricated single crystalpiezoelectric wafer may be directly bonded to the lower subassembly.Thereafter, apertures or recesses may be defined in the piezoelectricwafer by any suitable technique, such as ion milling, and metal may bedeposited (e.g., via evaporative deposition) in the apertures orrecesses to form electrodes 38, 40. The piezoelectric wafer anddeposited electrodes may then be processed (e.g., via grinding andpolishing) to a desired thickness that exposes the electrodes 38, 40 toform a piezoelectric layer 12 with electrodes 38, 40 extending throughthe entire thickness thereof. Thereafter, the second slow wavepropagation layer 14B and the second guided wave confinement structure16B may be sequentially deposited over the piezoelectric layer 12.

FIG. 13 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 12 , except that first and second Bragg mirrors areused instead of first and second fast wave propagation layers as thefirst and second guided wave confinement structures 16A, 16B.Fabrication of the device of FIG. 13 is substantially similar tofabrication of the device of FIG. 12 . Although FIGS. 12 and 13 depictMEMS guided wave devices with two-sided confinement, in alternativeembodiments the second (top) side confinement structure 16B may beomitted, optionally in combination with omission of the second slow wavepropagation layer 14B.

FIG. 14 illustrates a MEMS guided wave device including electrodes 38,40 in the form of an IDT embedded in a lower face of a single crystalpiezoelectric layer 12, with an optional slow wave propagation layer 14contacting the electrodes 38, 40 and the piezoelectric layer 12, and onesided guided wave confinement provided by a fast wave propagation layerserving as a guided wave confinement structure 16 according to oneembodiment of the present disclosure. The electrodes 38, 40 areasymmetrically arranged relative to a center of the piezoelectric layer12, and gaps 52 between the electrodes 38, 40 are filled withpiezoelectric material. To fabricate the MEMS guided wave device of FIG.14 , the guided wave confinement structure 16 and optional slow wavepropagation layer 14 may be deposited over a substrate 28 to form alower subassembly. Separately, recesses may be defined in a surface of asingle crystal piezoelectric wafer by any suitable technique, such asphotolithographic etching, and metal may be deposited in the recesses toform electrodes 38, 40. The piezoelectric wafer surface containing thedeposited electrode material may be planarized and polished and directlybonded to a planarized and polished surface of the slow wave propagationlayer 14. Thereafter, the exposed upper surface of the piezoelectricwafer may be processed (e.g., via grinding) to a desired thickness toyield the piezoelectric layer 12 with electrodes 38, 40 recessed in alower portion thereof.

FIG. 15 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 14 , except that a Bragg mirror is used instead of afast wave propagation layer as a guided wave confinement structure 16.Fabrication of the device of FIG. 15 is substantially similar tofabrication of the device of FIG. 14 .

FIG. 16 illustrates a MEMS guided wave device including a PPT 58 havingelectrode layers 54, 56 sandwiching a piezoelectric layer withalternating polarity piezoelectric regions 12A, 12B, and furtherincluding an optional slow wave propagation layer 14, a fast wavepropagation layer serving as a one-sided guided wave confinementstructure 16, and an optional carrier substrate 28 according to oneembodiment of the present disclosure. The electrode layers 54, 56 aresymmetrically arranged relative to a center of the piezoelectric layerconsisting of the alternating polarity piezoelectric regions 12A, 12B.To fabricate the MEMS guided wave device of FIG. 16 , the guided waveconfinement structure 16, the optional slow wave propagation layer 14,and the first electrode layer 54 may be deposited over the substrate 28to form a lower subassembly. A piezoelectric wafer may be directlybonded to the lower subassembly, preceded and optionally followed withappropriate planarization and/or polishing steps. In certainembodiments, alternating polarity piezoelectric regions 12A, 12B may bedefined (e.g., via liquid cell poling or e-beam writing) prior tobonding; in other embodiments, the alternating polarity piezoelectricregions 12A, 12B may be defined after bonding is complete. Thereafter,the second electrode layer 56 may be deposited over the alternatingpolarity piezoelectric regions 12A, 12B to form the PPT 58. Examples ofPPT structures and PPT fabrication techniques are disclosed in U.S. Pat.No. 7,898,158, which is hereby incorporated by reference herein.

FIG. 17 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 16 , except that a Bragg mirror is used instead of afast wave propagation layer as a guided wave confinement structure 16.Fabrication of the device of FIG. 17 is substantially similar tofabrication of the device of FIG. 16 .

FIG. 18 illustrates a MEMS guided wave device with a PPT similar to thedevice of FIG. 16 , but with addition of double sided confinement. Thedevice of FIG. 18 includes a PPT 58 having electrode layers 54, 56sandwiching a piezoelectric layer with alternating polaritypiezoelectric regions 12A, 12B, and further including optional slow wavepropagation layers 14A, 14B sandwiching the PPT 58. Additionally, firstand second fast wave propagation layers serving as first and secondguided wave confinement structures 16A, 16B provide two-sidedconfinement. An optional substrate 28 is disposed under the first guidedwave confinement structure 16A. The electrode layers 54, 56 aresymmetrically arranged relative to a center thickness of thepiezoelectric layer. Fabrication of a lower portion of the MEMS guidedwave device of FIG. 18 proceeds according to the steps employed infabricating the device of FIG. 16 , with the additional steps ofdepositing the second slow wave propagation layer 14B and the secondguided wave confinement structure 16B over the PPT 58.

FIG. 19 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 18 , except that Bragg mirrors are used instead offast wave propagation layers as the first and second guided waveconfinement structures 16A, 16B. Fabrication of the device of FIG. 19 issubstantially similar to fabrication of the device of FIG. 18 .

FIG. 20 illustrates a MEMS guided wave device including a segmentedsingle crystal piezoelectric layer 72 arranged between a continuouselectrode layer 64 and a segmented electrode layer 66 includingelectrode segments substantially registered with segments of thepiezoelectric layer 72. The device includes an optional slow wavepropagation layer 14, including a fast wave propagation layer serving asa one-sided wave confinement structure 16, and including an optionalcarrier substrate 28 according to one embodiment of the presentdisclosure. Segmentation of the segmented piezoelectric layer 72 andsegmentation of the segmented electrode 66 yields gaps 68 between thesegmented structures, and such segmentation is preferably performedafter the MEMS guided wave device is formed to facilitate registrationof the respective features. The electrode layers 64, 66 areasymmetrically arranged relative to a center of the segmentedpiezoelectric layer 72. To fabricate the MEMS guided wave device of FIG.20 , the guided wave confinement structure 16, the optional slow wavepropagation layer 14, and the continuous electrode layer 64 may bedeposited over the substrate 28 to form a lower subassembly. Thereafter,an appropriately planarized and polished piezoelectric wafer is bondedto the lower subassembly. An upper surface of the piezoelectric layermay be thinned and polished, followed by deposition of an upperelectrode layer. Thereafter, apertures may be formed in the upperelectrode layer and the piezoelectric layer (e.g., via one or morephotolithographic etching steps) to form segments of the segmentedelectrode layer 66 that are substantially registered with segments ofthe segmented piezoelectric layer 72. The guided wave device of FIG. 20may be used to generate a mixed wave that includes a vertical componentas well as a transverse component.

FIG. 21 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 20 , except that a Bragg mirror is used instead of afast wave propagation layer as a guided wave confinement structure 16.Fabrication of the device of FIG. 21 is substantially similar tofabrication of the device of FIG. 20 .

In alternate embodiments, the MEMS guided wave devices of FIGS. 20 and21 may be modified to at least partially fill (e.g., flush-fill incertain instances) gaps 68 between segments of the segmented electrodelayer 66 and the segmented piezoelectric layer 72. Additionally, oralternatively, in certain embodiments the MEMS guided wave devices ofFIGS. 20 and 21 may be modified to replace the continuous electrodelayer 64 with a (second) segmented electrode layer, to thereby providesymmetric arrangement between the electrodes and a center thickness ofthe piezoelectric layer.

FIG. 22 illustrates a MEMS guided wave device with a continuouselectrode layer 64 and a segmented electrode layer 66 includingelectrode segments substantially registered with segments of thepiezoelectric layer 72 similar to the device of FIG. 21 , but withaddition of double sided confinement. The device of FIG. 22 includes asegmented single crystal piezoelectric layer 72 arranged between acontinuous electrode layer 64 and a segmented electrode layer 66including segments substantially registered with segments of thepiezoelectric layer 72, with the device including optional slow wavepropagation layers 14A, 14B sandwiching the electrode layers 64, 66 andthe piezoelectric layer 72, and with gaps 68 between respective segmentsof the segmented electrode layer 66 and the segmented piezoelectriclayer 72 being at least partially filled (preferably completely filled)with slow wave propagation material of the second slow wave propagationlayer 14B. Additionally, first and second fast wave propagation layersserving as first and second guided wave confinement structures 16A, 16Bprovide two-sided confinement. An optional substrate 28 is disposedunder the first guided wave confinement structure 16A. The electrodelayers 64, 66 are asymmetrically arranged relative to a center thicknessof the piezoelectric layer 72. Fabrication of a lower portion of theMEMS guided wave device of FIG. 22 proceeds according to the stepsemployed in fabricating the device of FIG. 20 , with the additionalsteps of depositing the second slow wave propagation layer 14B over andbetween the segmented electrode layer 66 and the segmented piezoelectriclayer 72, followed by depositing the second guided wave confinementstructure 16B over the second slow wave propagation layer 14B.

FIG. 23 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 22 , except that Bragg mirrors are used instead offast wave propagation layers as the first and second guided waveconfinement structures 16A, 16B. Fabrication of the device of FIG. 23 issubstantially similar to fabrication of the device of FIG. 22 . AlthoughFIGS. 22 and 23 depict MEMS guided wave devices with two-sidedconfinement, in alternative embodiments the second (top) sideconfinement structure 16B may be omitted, optionally in combination withomission of the second slow wave propagation layer 14B. Additionally,although FIGS. 20 to 23 illustrate a continuous electrode layer 64, inalternative embodiments, such layers 64 may be replaced withdiscontinuous (e.g., segmented) electrodes.

FIGS. 24 and 25 illustrate MEMS guided wave devices substantiallysimilar to the devices of FIGS. 12 and 13 , except that double sidedconfinement is provided by guided wave confinement structures 16A, 16Bof mixed types. FIG. 24 illustrates a MEMS guided wave device includingelectrodes 38, 40 in the form of an IDT embedded in and extendingthrough the entire thickness of a single crystal piezoelectric layer 12,with slow wave propagation layers 14A, 14B sandwiching the electrodes38, 40 and the piezoelectric layer 12. Two sided guided wave confinementis provided by a fast wave propagation layer serving as first guidedwave confinement structure 16A arranged below the piezoelectric layer12, and by a Bragg mirror serving as a second guided wave confinementstructure 16B arranged above the piezoelectric layer 12. Slow wavepropagation layers 14A, 14B are additionally provided between therespective guided wave confinement structures 16A, 16B and thepiezoelectric layer 12. An optional substrate 28 is disposed under thefirst guided wave confinement structure 16A. The electrodes 38, 40 aresymmetrically arranged relative to a center thickness of thepiezoelectric layer 12. FIG. 25 is substantially similar to FIG. 24 ,except that two sided guided wave confinement is provided by a Braggmirror serving as first guided wave confinement structure 16A arrangedbelow the piezoelectric layer 12, and by a fast wave propagation layerserving as a second guided wave confinement structure 16B arranged abovethe piezoelectric layer 12. Fabrication of the MEMS guided wave devicesis substantially similar to the fabrication steps described inconnection with FIGS. 12 and 13 , followed by deposition of a secondslow wave propagation layer 14B and a second guided wave confinementstructure 16B over the piezoelectric layer 12. Although FIGS. 24 and 25depict MEMS guided wave devices with two-sided confinement, inalternative embodiments the second (top) side confinement structure 16Bmay be omitted, optionally in combination with omission of the secondslow wave propagation layer 14B.

Although specific embodiments with two sided confinement illustrated inthe drawings may include first and second (e.g., lower and upper) guidedwave confinement structures of the same type (e.g., both being fast wavepropagation layers or both being Bragg mirrors), it is specificallycontemplated that any embodiments illustrated herein may be modified toinclude guided wave confinement structures 16A, 16B of mixed types. Forexample, a Bragg mirror may be provided below a piezoelectric layer anda fast wave propagation layer may be provided above a piezoelectriclayer, or vice-versa, to provide two sided confinement.

In certain embodiments, electrodes may be arranged along differentplanes on surfaces of a piezoelectric layer, with one group ofelectrodes arranged within recesses defined in the piezoelectric layer.By recessing alternate electrodes, the periodicity can be reduced byhalf, thereby enabling higher operating frequencies. Additionally, bybringing electrodes of opposing polarity very close to one another,stronger acoustic wave excitation may be possible. If depth of therecesses defined in the piezoelectric layer is controlled, then spuriousresponse may be controlled. In certain embodiments, some or allelectrodes alternately defined on an upper surface and in recesses of apiezoelectric layer may be at least partially covered with functionallayer (e.g., a temperature compensation material or a slow wavepropagation material) having either a planar or an undulating topsurface. In certain embodiments, such a functional material may providetemperature compensation utility. MEMS guided wave confinement devicesincluding non-coplanar electrodes incorporating recessed electrodes aredescribed in connection with FIGS. 26 to 29 .

FIG. 26 illustrates a MEMS guided wave device including a piezoelectriclayer 12 defining multiple recesses 74, with electrodes 38, 40 in theform of an IDT deposited on the piezoelectric layer 12. A first group ofelectrodes 38 is arranged on an uppermost surface of the piezoelectriclayer 12, and a second group of electrodes 40 is arranged in therecesses 74, such that the first group of electrodes 38 is non-coplanarwith the second group of electrodes 40. The MEMS guided wave device ofFIG. 26 further includes an optional slow wave propagation layer 14below the piezoelectric layer 12, and fast wave propagation layerserving as a one sided guided wave confinement structure 16. An optionalsubstrate 28 is disposed under the guided wave confinement structure 16.The electrodes 38, 40 are asymmetrically arranged relative to a centerthickness of the piezoelectric layer 12. To fabricate the MEMS guidedwave device of FIG. 26 , a lower subassembly including the substrate 28,the guided wave confinement structure 16, and the slow wave propagationlayer 14 may be produced by sequential deposition steps. Followingplanarization and polishing of the slow wave propagation layer 14, aprefabricated single crystal piezoelectric wafer may be directly bondedto the lower subassembly. The piezoelectric wafer may be processed(e.g., via grinding and polishing) to a desired thickness. Thereafter,recesses may be defined in the piezoelectric wafer by any suitabletechnique, such as ion milling, and metal may be deposited (e.g., viaevaporative deposition) to form recessed electrodes 40 and non-recessedelectrodes 38.

FIG. 27 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 26 , except that a Bragg mirror is used instead of afast wave propagation layer as a guided wave confinement structure 16.Fabrication of the device of FIG. 27 is substantially similar tofabrication of the device of FIG. 26 .

FIGS. 28 and 29 illustrate MEMS guided wave devices similar to thedevices of FIGS. 26 and 27 , but with addition of double sidedconfinement.

The MEMS guided wave device of FIG. 28 includes a piezoelectric layer 12defining multiple recesses 74, with electrodes 38, 40 in the form of anIDT deposited on the piezoelectric layer 12. A first group of electrodes38 is arranged on a bulk upper surface of the piezoelectric layer 12,and a second group of electrodes 40 is arranged in the recesses 74, suchthat the first group of electrodes 38 is non-coplanar with the secondgroup of electrodes 40. Optional slow wave propagation layers 14A, 14Bsandwich the piezoelectric layer 12 and the electrodes 38, 40, withportions of the second slow wave propagation layer 14B further fillinggaps 68 above electrodes 40 and between electrodes 38. Additionally,first and second fast wave propagation layers serving as first andsecond guided wave confinement structures 16A, 16B provide two-sidedconfinement and sandwich the slow wave propagation layers 14A, 14B. Anoptional substrate 28 is disposed under the first guided waveconfinement structure 16A. The electrodes 38, 40 are asymmetricallyarranged relative to a center thickness of the piezoelectric layer 12.Fabrication of a lower portion of the MEMS guided wave device of FIG. 28proceeds according to the steps employed in fabricating the device ofFIG. 26 , with the additional steps of depositing the second slow wavepropagation layer 14B and the second guided wave confinement structure16B over the electrodes 38, 40 and the piezoelectric layer 12.

FIG. 29 illustrates a MEMS guided wave device substantially similar tothe device of FIG. 28 , except that Bragg mirrors are used instead offast wave propagation layers as the first and second guided waveconfinement structures 16A, 16B. Fabrication of the device of FIG. 29 issubstantially similar to fabrication of the device of FIG. 28 . AlthoughFIGS. 28 and 29 depict MEMS guided wave devices with two-sidedconfinement, in alternative embodiments the second (top) sideconfinement structure 16B may be omitted, optionally in combination withomission of the second slow wave propagation layer 14B.

In alternate embodiments, the MEMS guided wave devices of FIGS. 28 and29 may be modified to include mixed types of guided wave confinementstructures 16A, 16B, such as a Bragg mirror arranged below apiezoelectric layer and a fast wave propagation layer arranged above apiezoelectric layer, or vice-versa.

In certain embodiments, a MEMS guided wave device includes differingfirst and second thickness regions, a first group of electrodes arrangedon or adjacent to the first thickness region and configured fortransduction of a first lateral acoustic wave having a wavelength λ₁ inthe first thickness region, and a second group of electrodes arranged onor adjacent to the second thickness region and configured fortransduction of a second lateral acoustic wave having a wavelength λ₂ inthe second thickness region, wherein λ₁ differs from λ₂. At least oneguided wave confinement structure is configured to confine the firstlateral acoustic wave in the first thickness region, and configured toconfine the second lateral acoustic wave in the second thickness region.In this manner, multiple resonators of a single device may be used fortransduction of multiple widely different (e.g., octave difference)frequencies. Examples of multi-frequency MEMS guided wave devices areillustrated in FIGS. 30-36 .

FIG. 30 illustrates subassemblies of a MEMS guided wave device duringfabrication, following local thinning of a piezoelectric layer 82 priorto direct bonding of the piezoelectric layer to an underlying layer. InFIG. 30 , a lower subassembly includes a substrate 28, a fast wavepropagation layer serving as a guided wave confinement structure 16arranged over the substrate 28, and a temperature compensation layer 14arranged over the guided wave confinement structure 16. An uppersubassembly includes a piezoelectric wafer that has been locally thinned(e.g., via an appropriate technique such as ion milling or etching) toyield first and second thickness regions, wherein a thinner of the tworegions is filled with a temperature compensation material 84. Afterformation of the lower and upper subassemblies, mating surfaces may beprocessed (e.g., via planarization and polishing) and directly bonded.Thereafter, thickness of the piezoelectric layer 82 may be adjusted bygrinding and polishing the upper surface, and electrodes may bedeposited over the piezoelectric layer.

FIG. 31 illustrates a MEMS guided wave device produced with thesubassemblies illustrated in FIG. 30 , following wafer bonding,planarization/thinning of an outer surface of the piezoelectric layer(to yield a first thickness region 82-1 and a second thickness region82-2 of the piezoelectric layer) and deposition of electrodes 38-1,40-1, 38-2, 40-2 over the piezoelectric layer. A first group ofelectrodes 38-1, 40-1 (e.g., forming a first IDT) is arranged over thefirst thickness region 82-1, and a second group of electrodes 38-2, 40-2(e.g., forming a second IDT) is arranged over the second thicknessregion 82-2, with all electrodes 38-1, 40-1, 38-2, 40-2 beingsubstantially coplanar and arranged to asymmetric guided waveexcitation. As shown in FIG. 31 , a periodicity (or spacing) ofelectrodes within the first group of electrodes 38-1, 40-1 preferablydiffers from a periodicity of electrodes within the second group ofelectrodes 38-2, 40-2. Through addition of the temperature compensationmaterial 84 of the upper subassembly (shown in FIG. 30 ) to thetemperature compensation layer 14 of the lower subassembly, theresulting MEMS guided wave device includes a first temperaturecompensation layer thickness region 14-1 and a second temperaturecompensation layer thickness region 14-2, wherein each such region 14-1,14-2 has a different thickness. In operation of the MEMS guided wavedevice of FIG. 31 , laterally excited waves are stimulated in the firstand second thickness regions 82-1, 82-2 of the piezoelectric layer usingthe first group of electrodes 38-1, 40-1 and the second group ofelectrodes 38-2, 40-2, respectively, and the guided wave confinementstructure 16 (i.e., fast wave propagation layer) serves to confine thelaterally excited waves in the first and second thickness regions 82-1,82-2.

FIGS. 32 and 33 are substantially similar to FIGS. 30 and 31 , exceptthat a Bragg mirror is provided instead of a fast wave propagation layeras the guided wave confinement structure 16. Fabrication of the MEMSguided wave confinement structure of FIG. 33 is substantially similar tothe fabrication steps described in connection with the MEMS guided waveconfinement structure of FIG. 31 , except for replacement of the fastwave propagation layer with a Bragg mirror to serve as the guided waveconfinement structure 16.

FIG. 34 illustrates a MEMS guided wave device including first and secondthickness regions 82-1, 82-2 of a piezoelectric layer that are overlaidwith a first group of electrodes 38-1, 40-1 and a second group ofelectrodes 38-2, 40-2, respectively. Each group of electrodes 38-1,40-1, 38-2, 40-2 is substantially coplanar. The first and secondthickness regions 82-1, 82-2 of the piezoelectric layer overlie atemperature compensation layer including a first temperaturecompensation layer thickness region 14-1 and a second temperaturecompensation layer thickness region 14-2. Thereunder, a guided waveconfinement structure 16 in the form of a fast wave propagation materialincluding a first guided wave confinement thickness region 16-1 and asecond guided wave confinement thickness region 16-2 is provided. Theguided wave confinement structure 16 may optionally be supported by anunderlying substrate 28. Ultimately, the first group of electrodes 38-1,40-1 (e.g., forming a first IDT) is arranged over a first thicknessregion 82-1 of the piezoelectric layer, a first temperature compensationlayer thickness region 14-1, and a first guided wave confinementthickness region 16-1. Similarly, the second group of electrodes 38-2,40-2 (e.g., forming a second IDT) is arranged over a second thicknessregion 82-2 of the piezoelectric layer, a second temperaturecompensation layer thickness region 14-2, and a second guided waveconfinement thickness region 16-2. In this manner, wave propagationcharacteristics, temperature compensation characteristics, and guidedwave confinement characteristics may be selected for each of the firstgroup of electrodes 38-1, 40-1 and the second group of electrodes 38-2,40-2.

The MEMS guided wave device of FIG. 34 may be fabricated from multiplesubassemblies. A lower subassembly may be formed by depositing a guidedwave confinement structure (e.g., a fast wave propagation material) on asubstrate 28, followed by locally thinning the guided wave confinementstructure, depositing a temperature compensation material in the locallythinned area, and depositing further temperature compensation materialover the entire surface. An upper subassembly may be formed by locallythinning a prefabricated piezoelectric wafer, and depositing temperaturecompensation material in the locally thinned area. After appropriateplanarization and polishing of mating surfaces of the respectivesubassemblies, the upper and lower subassemblies may be directly bondedto one another. Thereafter, thickness of the piezoelectric layer 82 maybe adjusted by grinding and polishing, and groups of electrodes 38-1,40-1, 38-2, 40-2 may be deposited on an upper surface thereof.

FIGS. 35 and 36 illustrate MEMS guided wave devices in which apiezoelectric layer includes different thickness regions withnon-coplanar top surfaces over which different groups of electrodes arearranged. Referring to FIG. 35 , a MEMS guided wave device includesfirst and second thickness regions 82-1, 82-2 of a piezoelectric layerthat are overlaid with a first group of electrodes 38-1, 40-1 (e.g.,forming a first IDT) and a second group of electrodes 38-2, 40-2 (e.g.,forming a second IDT), respectively. Top surfaces of the first andsecond thickness regions 82-1, 82-2 are non-coplanar, such that thefirst group of electrodes 38-1, 40-1 is non-coplanar with the secondgroup of electrodes 38-2, 40-2. The first and second thickness regions82-1, 82-2 of the piezoelectric layer overlie a temperature compensationlayer 14. Thereunder, a guided wave confinement structure 16 in the formof a fast wave propagation material is provided. The guided waveconfinement structure 16 may optionally be supported by an underlyingsubstrate 28. The MEMS guided wave device of FIG. 35 may be fabricateddepositing a guided wave confinement structure 16 (e.g., a fast wavepropagation material) on a substrate 28, and depositing a temperaturecompensation layer 14 over guided wave confinement structure 16. Afterappropriate planarization and polishing of a mating surface of thetemperature compensation layer 14 and a mating surface of aprefabricated piezoelectric wafer, such surfaces may be directly bonded.Thereafter, the piezoelectric wafer may be locally thinned, such as byion milling, to define the first and second thickness regions 82-1,82-2, and groups of electrodes 38-1, 40-1 and 38-2, 40-2 may bedeposited thereon.

Thus, a method of fabricating a micro-electrical-mechanical system(MEMS) guided wave device including a single crystal piezoelectricmaterial with different thickness regions includes local thinning of asingle crystal piezoelectric layer to define first and second thicknessregions that differ in thickness. The locally thinned piezoelectriclayer is bonded on or over an underlying layer (e.g., at least one of(i) a fast wave propagation layer; (ii) a Bragg mirror, or (iii) asubstrate) to provide an internally bonded interface. Such bonding maybe performed using wafer bonding techniques known in the art. First andsecond groups of electrodes are defined on or adjacent to the firstthickness region and the second thickness region, respectively, fortransduction of a first lateral acoustic wave having a first wavelengthλ₁ in the first thickness region, and for transduction of a secondlateral acoustic wave having a second wavelength λ₂ in the secondthickness region. One or more surfaces of the piezoelectric layer may beplanarized prior to bonding (e.g., as a bonding preparation step),and/or planarized after bonding (e.g., to adjust thickness of thepiezoelectric layer). Preferably, a temperature compensation layer maybe provided below the piezoelectric layer, wherein in certainembodiments, the temperature compensation layer may include a firsttemperature compensation layer thickness region and a second temperaturecompensation layer thickness that differ from one another. In certainembodiments, additional temperature compensation material may bedeposited on or over a surface of at least one of the first thicknessregion or the second thickness region.

Upon reading the following description in light of the accompanyingdrawing figures, those skilled in the art will understand the conceptsof the disclosure and will recognize applications of these concepts notparticularly addressed herein. Those skilled in the art will recognizeimprovements and modifications to the preferred embodiments of thepresent disclosure. All such improvements and modifications areconsidered within the scope of the concepts disclosed herein and theclaims that follow. Any of the various features and elements asdisclosed herein may be combined with one or more other disclosedfeatures and elements unless indicated to the contrary herein.

What is claimed is:
 1. A method of fabricating amicro-electrical-mechanical system (MEMS) guided wave device, the methodcomprising: locally thinning a single crystal piezoelectric materiallayer to define a first thickness region and a second thickness region,wherein a thickness of the first thickness region differs from athickness of the second thickness region; bonding the locally thinnedsingle crystal piezoelectric material layer on or over an underlyinglayer to provide a bonded interface; defining a first plurality ofelectrodes arranged on or adjacent to the first thickness region andconfigured for transduction of a lateral acoustic wave having awavelength λ₁ in the first thickness region; and defining a secondplurality of electrodes arranged on or adjacent to the second thicknessregion and configured for transduction of a lateral acoustic wave havinga wavelength λ₂ in the second thickness region.
 2. The method of claim1, further comprising planarizing at least one surface of the singlecrystal piezoelectric material layer prior to the bonding of the locallythinned single crystal piezoelectric material layer on or over theunderlying layer.
 3. The method of claim 1, further comprisingdepositing a temperature compensation material on a surface of at leastone of the first thickness region or the second thickness region.
 4. Themethod of claim 3, wherein the temperature compensation materialcomprises a first temperature compensation layer thickness proximate tothe first thickness region, and comprises a second temperaturecompensation layer thickness proximate to the second thickness region,wherein the second temperature compensation layer thickness differs fromthe first temperature compensation layer thickness.
 5. The method ofclaim 1, wherein the first plurality of electrodes comprises a firstinterdigital transducer, and the second plurality of electrodes definesa second interdigital transducer.
 6. The method of claim 1, wherein thefirst plurality of electrodes is substantially coplanar with the secondplurality of electrodes.
 7. The method of claim 1, wherein the firstplurality of electrodes is non-coplanar with the second plurality ofelectrodes.
 8. The method of claim 1, wherein the underlying layercomprises at least one of a fast wave propagation layer or a Braggmirror.
 9. The method of claim 1, wherein the underlying layer comprisesa substrate.
 10. The method of claim 1, wherein the underlying layercomprises a substrate in combination with at least one of a fast wavepropagation layer or a Bragg mirror, wherein the at least one of a fastwave propagation layer or a Bragg mirror is arranged between thesubstrate and the single crystal piezoelectric material layer.
 11. Amethod of fabricating a micro-electrical-mechanical system (MEMS) guidedwave device comprising a segmented single crystal piezoelectric materiallayer, the method comprising: forming a first guided wave confinementstructure over a substrate; forming a first electrode layer over thefirst guided wave confinement structure; providing a single crystalpiezoelectric material layer over the first guided wave confinementstructure; forming an upper electrode layer over the single crystalpiezoelectric material layer; and defining a plurality of aperturesextending through the upper electrode layer and the single crystalpiezoelectric material layer to form segments of the upper electrodelayer that are substantially registered with segments of the singlecrystal piezoelectric material layer.
 12. The method of claim 11,wherein the defining of the plurality of apertures extending through theupper electrode layer and the single crystal piezoelectric materiallayer comprises etching portions of the upper electrode layer and thesingle crystal piezoelectric material layer.
 13. The method of claim 11,wherein the providing of the single crystal piezoelectric material layerover the first guided wave confinement structure comprises bonding apiezoelectric layer over the guided wave confinement structure.
 14. Themethod of claim 11, further comprising providing a slow wave propagationlayer between the first guided wave confinement structure and the singlecrystal piezoelectric material layer.
 15. The method of claim 11,wherein the first guided wave confinement structure comprises at leastone of a fast wave propagation layer or a Bragg mirror.
 16. The methodof claim 11, wherein the first electrode layer is continuous.
 17. Themethod of claim 11, wherein the plurality of apertures extends throughthe first electrode layer to form segments of the first electrode layer.18. The method of claim 11, further comprising at least partiallyfilling gaps between adjacent segments of the upper electrode layer andbetween adjacent segments of the single crystal piezoelectric materiallayer with a slow wave propagation material.
 19. The method of claim 18,further comprising forming a second guided wave confinement structureover the slow wave propagation material.
 20. The method of claim 19,wherein the second guided wave confinement structure comprises at leastone of a fast wave propagation layer or a Bragg mirror.